Metal bromides, such as sodium bromide, potassium bromide, magnesium bromide, and calcium bromide etc., are important commercial materials. New uses for even the oldest of these compounds are being introduced and demand for them continues to grow. For example, the use of salts such as sodium bromide and calcium bromide in the removal of mercury from the flue gas of coal burning power plants is growing at a rapid pace. While the preparation of many bromine containing compounds such as metal salts is conceptually simple, there are significant challenges in developing new and more efficient industrial processes for their preparation that use less energy, produce less waste, provide cleaner products, reduce costs, etc.
The preparation of metallic bromides by reaction of an alkali or alkaline earth metal compound (e.g. a compound of sodium, potassium, calcium, and the like) with bromine in the presence of a reducing agent (e.g., urea, cyanamide, ammonium carbonate, ammonium bicarbonate, formamide, carbamates, ammonium cyanide and formic acid, oxalic acid, and their salts) has long been known. Essentially the process involves fast reaction between the reducing agent and bromine producing HBr, which then reacts with the alkali or alkaline earth metal compound.
British specification No. 285,915 discloses the preparation of calcium bromide by reacting a “non-acid” calcium compound (e.g. calcium oxide, hydroxide, and/or carbonate) with bromine in the presence of a reducing agent which is converted to gas and/or water. The reducing agent insures that substantially no bromates or hypobromites are formed as by-products. This patent describes several reaction sequences, among which are addition of the metal salt to a reaction medium comprising reducing agent, bromine, and water (Example I); addition of an aqueous solution of metal salt and reducing agent to an aqueous bromine reaction medium (Example II); and addition of bromine to a reaction medium comprising the metal salt, reducing agent, and water (Example (III).
U.S. Pat. Nos. 1,863,375 and 2,007,758 disclose a process for preparing metal bromides employing ammonia to retard the formation of bromate and hypobromite. The U.S. Pat. No. 1,863,375 discloses the recirculation of an aqueous ammonia containing metal salt solution through a tower absorber in which it is exposed to bromine vapor. U.S. Pat. No. 2,007,758, relates to the same general process, but is specifically concerned with means for recovering the spent ammonia evolved from the reaction mixture.
U.S. Pat. No. 2,269,733 discloses the reaction of an alkali or alkaline earth metal compound with bromine in the presence of one of a variety of reducing agents. Several alternative reaction sequences are described including the simultaneous addition of bromine and metal salt to a mother liquor, with an excess of reducing agent preferably being employed. Alternatively, a two step process is disclosed in which ammonia and bromine are first reacted in the presence of mother liquor to form ammonium bromide, with the metal salt thereafter being added together with additional bromine.
U.S. Pat. No. 4,083,942 discloses a process wherein a mixture of metal salt and reducing agent in water is first prepared and to this mixture is added in a stepwise manner alternate portions of bromine and the metal salt. A wide variety of reducing agents useful in the above general process are disclosed in the art including, e.g., U.S. Pat. No. 4,248,850 which discloses ammonia, ammonium salts, formic acid, formate salts, formamide, and formaldehyde, and U.S. Pat. No. 4,514,374, which discloses the use of lower alcohols.
Variations of the above general process, including the preparation of other halides, e.g., chlorides, and the use of other metals, e.g., zinc, can be found in, e.g., U.S. Pat. No. 6,117,408, U.S. Pat. No. 6,036,937, and U.S. Pat. No. 7,087,209.
There is still a need for an improved bromide transfer process capable of forming bromine containing compounds that efficiently uses all reagents, creates less waste, uses less energy and provides high purity material.